# Strange Wheatstone bridge

Hi,
I have salvaged pressure sensor and I know little about it. After some internet brainstorming I started to experiment. There are 4 wires from the sensor, it looks like no "smartness" inside - it is possible to measure resistance between each two wires and when applying some voltage it follows Ohm's law. After more experimenting I decided it is in strange (for me) Wheatstone bridge configuration. But it does not look like schematic on Wiki but more like exercise from All About Circuits:

Interestingly R3 is smallest from all the resistors: R1 ~ R2 ~ R4 ~ R5 ~ 1200 Ohm while R3 ~300 Ohm. I tried to measure (amplified) voltage over R3 and I got some reaction on increasing pressure. But I don't know if I am using it in the right way - I would expect R3 reduces sensitivity. I guess it is meant to reduce interference or long cables problem (current should be measured instead of voltage???). Do you have any idea?

That is called an unbalanced wheatstone bridge. A standard wheatstone bridge does not have R3 and R1 is an unknown varying value.

Well OK. But googling "unbalanced Wheatstone bridge" helps me to calculate the equivalent resistance but no hint how to use it as sensor and why it is better than bridge without R3. Or I understand it wrong? It may be 90° rotated but then I have no idea what would reason for using R3 be - except for higher current draw and increasing problems from wires resistance?
On second thought if R3 was "parallel" to the bridge it would lead to higher current flowing to the bridge and so less noise (more stable voltage at bridge input)???

No clue. I tried to search on when to use the standard wheatstone bridge versus the unbalanced wheatstone bridge and came up blank.

That is called an unbalanced wheatstone bridge. A standard wheatstone bridge does not have R3 and R1 is an unknown varying value.

Not exactly. As drawn it is a plain ordinary bridge circuit and I would say it is balanced if R1, R2, R4, and R5 are the same value.

A Wheatstone bridge has a meter (which has some resistance) where R3 is located in this drawing.

Don

The circuit as drawn in post #1 is called "An Unbalanced Wheatstone Bridge". Check the EE textbooks and google. I recalled seeing in school (20 years ago) as one of those examples of tricky need-to-understand-the-topic problem solving circuits.

In the standard 4-resistor Wheatstone Bridge, balanced has nothing to do with the value of the resistors. In fact, it is very usual and common for the resistors to be different values. It depends on the the value of the varying resistor (typically R1), and the desired operating point formed by the difference of the voltages of the resistor dividers R1-R4 and R2-R5.

The issue is, that I cannot find any reference on when, how, or why you would use the unbalanced wheatstone bridge as drawn in post #1. I have lots of applications, and have implemented or designed plenty of 4-resistor Wheatstone bridge circuits. But have never seen nor used a 5-resistor Unbalanced Wheatstone bridge , nor do I know or can find out when you would want to. There must be a use or reason for the circuit in post #1, but I can't find it.

How did you arrive at the values of R1, R2, R3 etc.?

Measuring the resistance between any two wires does not give the value of any one of the resistances shown in the circuit in post #1.

Instead, each resistance measurement corresponds to a complicated series/parallel combination of all of the resistors.

I will leave it to the readers to refer to EE Textbooks and/or Google to determine for themselves how to differentiate a Wheatstone bridge from other similar configurations and how to determine if it is balanced or not.

The statement by jremington above is correct.

Don

jremington:
How did you arrive at the values of R1, R2, R3 etc.?

Measuring the resistance between any two wires does not give the value of any one of the resistances shown in the circuit in post #1.

Instead, each resistance measurement corresponds to a complicated series/parallel combination of all of the resistors.

I guessed them If the resistors were those values I would roughly get resistances I measured. Since there is symetry I think values close to my guess are only possibility (if the confuguration is equivalent to this circut and I have good reasons to think it is). If you want more explanation what I have measured and how I can write it but I thought it would complicate the situation with no benefit.

Take a look at this information about a resistive-bridge pressure sensor which is probably what you are dealing with. What you need is on the first page.

Note that there is no physical device representing "R3" within the sensor itself. If you consider the differential amplifier to be a "meter" then you would have a Wheatstone bridge configuration. If you were able to apply some specific amount of pressure such that the "meter" reads zero then the bridge would be "balanced".

Don

If you want more explanation what I have measured and how I can write it but I thought it would complicate the situation with no benefit.

The advantage of posting your measurements is that there are other possibilities to explain them. I doubt that the circuit posted is correct, but wish you good luck with the project.

jremington:
The advantage of posting your measurements is that there are other possibilities to explain them. I doubt that the circuit posted is correct, but wish you good luck with the project.

OK, I named the nodes to make it easier.

I am not at home so I cannot make more measurements. I will write what I recall - it may be a bit inconsistent. But it is all I currently have.
At first I measured resistance between two wires by DMM. Resistance was symetric: from A to B same as from B to A (and the same for all pairs). The values were roughly:
AB ~ BC ~ CD ~ DA ~ 1200 Ohm
BD ~ 300 Ohm
AC ~ 2400 Ohm
(Now I noticed it would lead to R1 ~ R2 ~ R4 ~ R5 ~ 2400 Ohm, not 1200 as I wrote in OP).
Then I applied voltage between 2 points and measured current. With changing voltage the current changed roughly following Ohm's law (I used my crude Arduino "oscilloscope").
Then I fixed voltage to 2V and applied to the wires measuring one of them with my "osciloscope" (measured voltages were not exact, just estimate).
C = 2V, A = GND -> B~D~1V
B = 2V, D = GND -> C~A~1V
B = 2V, C = GND -> A, D a bit less than 2V (not sure if the same voltage or slightly different)
B = A = 2V, C = GND -> D less than 2V but higher then in previous
B = D = 2V, C = GND -> A exactly 2V.
I concluded all points have direct connection except for A and C.
EDIT: I am not sure what this does mean. More importantly I am not sure if I remember it correctly. I was quite sure it is as drawn but I am not so sure now. I will check it again when I get home next week.

This will produce approximately those measurements and makes sense:

Edit: fixed labels. Thanks, MarkT

You switched C and D, but yes, that looks plausible.

Active investigation with power applied would clinch it.

jremington:
This will produce approximately those measurements and makes sense:

Yes, this makes more sense. It is equivalent to my original schematics, right? Is there a way to decide which configuretion is used?
Why the 1k resistors R1 and R2 (in your schematic)? Is it just to reduce current through the bridge but reducing output voltage difference as well or is there some other advantage of them? Such as less current = less heating = better accuracy or something like that?

It is equivalent to my original schematics, right?

No.

Why the 1k resistors R1 and R2 (in your schematic)?

See the link in reply #9.

jremington:
No.

By equivalent I mean measuring something only at points A, B, C and/or D will yield the same results. If I use in my original schematic R1 = R2 = R4 = R5 = 2300 Ohm and R3 = 345 Ohm I get the same resistances as in your schematic. So where is the difference and how I can confirm your schematic is the right one?

jremington:
See the link in reply #9.

I read it but did not find much information about this topic. R1 and R2 are named Rtop and Rbottom. They are shown only on Figure 4 and not mentioned in the text. The caption of the figure hints they are used to control common mode voltage of the differential points. But since they are both the same value the common mode voltage is 1/2 supply voltage both with and without them.

I can think of several reasons (e.g. reduce current draw) to have the resistors at the top and bottom, but only the designer would know for sure.

It does not make any sense to have a low value resistor across the middle, as that would severely affect the sensitivity of the bridge, in response to changes in the branch resistances.

But since they are both the same value the common mode voltage is 1/2 supply voltage both with and without them.

The caption for figure 4 states: “Adjust VCM by adding RTOP and/or RBOTTOM” Doesn’t the “and/or” imply that they aren’t necessarily the same value?

Don

It does not make any sense to have a low value resistor across the middle, as that would severely affect the sensitivity of the bridge, in response to changes in the branch resistances.

That's why there's a differential amplifier (most likely with a very high input impedance) connected 'across the middle', not a resistor. EDIT: Just as it says below figure 2.

Don